This invention relates to drop marking devices such as ink jet printers and the like. Such devices come in various forms including single drop stream devices and multiple drop stream devices such as the binary array type. Drop marking systems generally employ electrically conductive inks which are supplied to one or more nozzles which have associated therewith a piezoelectric crystal. The ink is forced through the nozzle and, by virtue of the piezoelectric device, discrete drops are formed as the ink leaves the nozzle. The drops next pass through a charging device, such as a charge ring, which selectively imparts electrical charges to them. That is, some drops are charged while others are not. The drops which are charged may have charges of different magnitudes placed thereon.
As the drops continue along their initial path, they enter an electric field formed by a pair of deflection plates. Typically, these plates are charged to a high voltage by a power supply. As the drops enter the deflection field formed between the plates, the charged drops are deflected from their original flight path by an amount proportional to the charge which they carry. Thus, uncharged drops are not deflected while highly charged drops are deflected a significant amount. In the usual case, charged drops will be deflected on to a substrate to be marked while uncharged drops are collected for return to an ink reservoir. Additional detail concerning typical ink drop marking systems can be found in U.S. Pat. Nos. 4,121,222, 4,319,251 and 4,555,712, hereby incorporated by reference.
A problem with ink jet printers of the type described occurs due to the necessity for producing the electric field for deflecting the drops. Typically, this electric field is produced by connecting one deflection plate or electrode to a high voltage power supply while connecting the second electrode to ground. Alternatively, a high voltage power supply of one polarity is connected to a first deflection electrode, and a power supply of the opposite polarity is connected to the other electrode.
The high voltage power supplies required for these purposes are responsible for several problems in the safety and performance area of the printing devices. First, the power supplies produce electrical noise which disturbs the proper functioning of the other electrical circuits associated with such printing systems, including the computer systems which determine the drop marking patterns to be placed on the substrate and associated functions. Further, arcing between deflection electrodes or elsewhere in the high voltage circuits causes poor print quality and other operational problems. With respect to safety, the power supply energy released during arcing between the deflection electrodes can create a fire hazard particularly where the inks used contained flammable solvents as is often the case. Finally, the high voltage power supplies present a possible safety hazard due to the potential for electrical shock.
Accordingly, it is desirable to eliminate the need for high voltage supplies in drop marking systems while retaining the ability to selectively deflect ink drops on to a substrate for marking purposes. Such a system would eliminate the noise and safety problems inherent in the use of high voltage power supplies to create the deflection fields.
It is accordingly an object of the present invention to provide a device which can provide a deflection field for charged drops, which device does not require the use of a high voltage power supply to maintain the deflection field.
Another object of the invention is to provide a method for deflecting charged drops without the need for employing high voltage power supplies.
A further object of the invention is to provide a deflection field of the type described by use of one or more electrets, whereby a deflection field is maintained without the need for external power supplies.
Another object of the invention is to provide an improved drop marking system in which electrical noise is reduced and accuracy of drop placement is maintained.
A further object of the invention is to provide a drop marking system which is reliable and reduced in cost and complexity.
These and other objects of the invention will be apparent from the following detailed description.